1. Field of the Invention
This invention relates to content addressable memory (CAM) to store the address location for the defective bit in memory redundancy scheme and more particularly to content addressable memories with multiple power voltage levels.
2. Description of the Related Art
A conventional content addressable memory (CAM) is composed of an array of stored words with each word in the array having an associated comparator. When a comparand data is presented to the CAM, a simultaneous compare operation is performed between the comparand and all the words in the array. If the comparand and a word in the CAM are matched, then a match signal is generated. A user can program a CAM by writing data into the array.
One application of a CAM is to provide for access to redundant addresses in an electrically programmable read only memory (EPROM). The CAM compares addresses for the EPROM to a set of addresses that are known to have manufacturing defects. If an address to the EPROM matches the contents of the CAM, then the address can be converted to a redundant row or column address for the spare elements of the EPROM, which provides redundancy for the EPROM as fault correction. The CAM and the EPROM can be fabricated together onto a single integrated circuit.
A conventional CAM operates from a single power supply voltage, which is typically a 5 volt V.sub.cc power supply. This invention is related to low voltage EPROM design, which the operating V.sub.cc is typically 3.3 v with a range from 2.6 v to 3.6.
If a CAM is fabricated on the same integrated circuit with an EPROM, then the CAM must operate properly when the EPROM on the integrated circuit is programmed with a conventional programmer. To program an EPROM using traditional programming equipment, a programming voltage V.sub.pp of about 12 volts is applied and the V.sub.cc power supply is raised to 6.25 volts. If a CAM normally operates at a V.sub.cc power supply level of 5 volts, then the CAM circuit V.sub.cc operation window can accommodate raising the voltage from 5 volts to 6.25 volts.
If a conventional CAM design is operated at a V.sub.cc power supply of 3.3 volts for the portable systems, without sacrificing power and speed, the CAM circuit V.sub.cc operation window does not have the range to accommodate raising the V.sub.cc power supply for the CAM from 3.3 volts to 6.25 volts, when the EPROM is programmed with a conventional programmer.
One approach to solve this problem would be to design the EPROM to be programmed at lower voltages; however, this approach would preclude the use of conventional EPROM programming equipment, which requires that high voltages be used for programming the EPROM. Changing the EPROM programming equipment to operate at a lower voltage would be expensive and would have required new process/device technology development.
A CAM must be reset or initialized to properly operate. For a conventional CAM operating at a V.sub.cc power supply of 5 volts, or a V.sub.cc power on reset is used to reset the CAM. The V.sub.cc power on reset does not alter the array of words stored in the CAM, but rather initializes the readout of the CAM to the proper state so that the CAM will operate properly.
A CAM operating at a low V.sub.cc power voltage of 3.3 volts does not function properly when programming the EPROM, because of the raised V.sub.cc power voltage level of 6.25 volts applied to the circuits is beyond the circuit V.sub.cc operation window of the CAM.
Accordingly, there is a need in the art for a system that allows a CAM to operate at a power voltage level of 2.6 to 3.6 volts to reduce power dissipation and at a higher voltage during programming of an electrically programmable memory. There is also a need for a system that resets a CAM for proper operation with multiple power supply levels. There is also a need for a system for CAM that provides access to redundant addresses of an electrically programmable read only memory (EPROM) and that allows programming of the EPROM with traditional programming equipment.